1. Field of the Invention
The present invention relates generally to an improved technique for measuring phase noise, and more particularly pertains to phase noise measurements utilizing a frequency down conversion/multiplier and direct spectrum measurement technique that uses a combination of a phase detector method and a direct spectrum phase noise measurement approach. The subject invention is particularly useful for field test environments where laboratory instrumentation is normally not available and fast and accurate phase noise measurements are required.
2. Discussion of the Prior Art
As the performance of microwave radar and communication systems advances, certain system parameters take on increased performance. One of these parameters that must be measured is the spectral purity of microwave signal sources. In the prior art, many techniques for measuring spectral purity have used complex dedicated instrumentation, often cumbersome in both size and operation. The broadening focus on spectral purity has created a requirement for phase noise measuring techniques which provide the high performance necessary for many environments requiring high precision such as the airborne radar environment.
The most effective way of determining the spectral purity of a signal source is by measuring its Single Side Band phase noise (SSB phase noise), or simply called phase noise.
One particular application wherein phase noise measurements are important is in doppler radar systems which determine the velocity of a target by measuring the small shifts in frequency that the return echo signals have undergone. In actual systems, however, the return signal is much more than just the target echo and includes a large "clutter" signal from the large stationary earth. If the clutter return signal is decorrelated by the delay time difference, phase noise from a local oscillator can partially or even totally mask the target signal. Thus phase noise can set the minimum signal level which must be returned by a target in order to be detectable.
Caldwell et al. U.S. Pat. No. 4,748,399 is similar in several respects to the present invention, but several significant differences exist therebetween.
The present invention is specifically designed for field measurements of phase noise, and special considerations were taken into account with respect to the size and weight of the overall implementation. Several alternative techniques were analyzed and discarded for the same reason as they were too big or too heavy for field applications.
The Caldwell multichannel phase noise measurement system requires two sources to perform a measurement which implies several restrictions. Two Units Under Test (UUTs) are required for a phase noise measurement. This measurement is possible only if both UUTs are multichannel units, which is important considering that the first Intermediate Frequency (IF) is the difference between both channels. Both UUTs cannot have the same frequency, in which case the output of the first mixer would be a base band signal rather than an IF signal. Programmability is required (either manual or remote) to obtain the proper IF signal.
An extra 3 dB error in the measurement has to be taken into consideration if the phase noise of both sources are similar, Also, if the measured phase noise does not meet the required specifications, a third UUT would be required to resolve the ambiguity between the two UUTs being tested.
The subject invention eliminates this problem by using a commercially available, ultra low phase noise source with a fixed center frequency as a second source rather than a second UUT. This yields an advantage of selecting the first IF and phase noise performance according to the UUT requirements. The present invention utilizes a 10 dB margin between the two phase noise sources, which makes the noise contribution of the fixed source less than half a dB, acceptable for field testing. Also, a fixed second source eliminates the need for programmability of the second source, making the field test implementation easier and lighter. If the frequency of interest or phase noise level requirements changes, the fixed source is replaced accordingly.
Another important difference between both approaches is the absence of lowpass filters in the Caldwell system. The approach of the present invention uses lowpass filters after the first mixer and also after each of the two frequency doublers. The first mixer is used as a down converter of the input signal to an Intermediate Frequency (IF) in which all other mixer outputs are considered to be spurious signals and discriminated against by the lowpass filter at its output. The subject invention uses a bandpass filter centered at the IF value so the measurement is not affected (desensitized) by spurious signals. With the same criteria, bandpass filters are used after each doubler.
A third and probably the most significant difference is the final approach used to measure the phase noise. Caldwell et al. utilizes a commercially available frequency synthesizer with a phase lock loop to measure the phase noise in a phase detector method. The UUT signal is down converted into a base band signal, and the final mixer is used as a phase detector. The purpose of the phase lock loop is to maintain a 90.degree. phase difference between the UUT signal and the output of the frequency synthesizer. This is important considering that the phase detector has its highest sensitivity when both signals are in quadrature.
In contrast thereto, the approach of the subject invention uses a combination of a phase detector for the down conversion portion and a direct spectrum method for the phase noise measurement technique. The manner in which the phase noise is measured is substantially different from Caldwell et al. First, the frequency synthesizer and phase lock loop is not required. The UUT signal (down converted, filtered and multiplied up) is sent directly to a spectrum analyzer. Because the phase noise of the signal has already been increased by a known factor, the phase noise can be accurately measured by the spectrum analyzer.
As mentioned hereinabove, the present invention is tailored to field measurement applications in which the utilization of a frequency synthesizer for the last down conversion stage is not recommended due to size and weight limitations. Moreover, the phase lock loop section of the phase detector method requires lengthy and complex calibration routines that makes the phase noise measurement very slow.
Another important factor is that the phase detector method of Caldwell et al. creates a base band signal with equivalent phase noise to that of the UUT. The frequency spectrum analyzer of Caldwell et al. must measure this signal from DC to the highest offset frequency of interest (normally around 1 MHz). This creates problems in field applications as normally field frequency spectrum analyzers are wideband, and the lowest frequency they can measure is around 100 Hz. This implies the use of a second low frequency spectrum analyzer just for the phase noise measurement. The approach of the present invention is not affected by this limitation. The UUT signal remains as an IF signal that falls in the most sensitive range of the spectrum analyzer. The sensitivity of the measurement is determined by the phase noise of the spectrum analyzer Local Oscillator (LO), and the UUT phase noise has been increased over the phase noise of the LO so that a direct spectrum analyzer measurement is acceptable.